1. Technical Field
This application relates to ultrasound imaging systems, for instance medical ultrasound diagnostic imaging systems and, in particular, to processing return or echo signals in ultrasound imaging systems.
2. Description of the Related Art
Ultrasound imaging systems employ transducer arrays to produce and transmit ultrasound into a body, tissue or other material. The transducer arrays also receive ultrasound returns or echoes and produce analog transducer element voltage signals which are induced at the transducer array by the received ultrasound returns or echoes. Ultrasound imaging systems typically use amplifiers to amplify the analog transducer element voltage signals before digitization. The analog amplification may vary with imaging depth (i.e., time gain compensation or control, i.e., TGC) to compensate for attenuation of ultrasound with depth.
Ultrasound imaging systems typically employ analog-to-digital converters (ADCs) to digitize the amplified analog transducer element voltage signals. Often a separate ADC is used for each analog channel, the ADCs mapped to the transducer elements of the transducer array. Appropriate focus delays may be applied before the digitized ADC output values from the channels are summed to form beams that are ultimately used to produce image data.
To reduce system cost and power consumption, the ADCs often limit the number of bits (e.g., 12 bits) used to digitize the transducer element voltage signals. The amplifiers are typically set so that a peak signal input to the ADC is close to a maximum range of the ADC (i.e., ADC output is close to a maximum possible digital value) in order to best discriminate the ultrasound signal from noise and to make maximum use of the dynamic range of the ADC (i.e., use as many of the available digital values as possible to represent the varying ultrasound signal). However, if saturation occurs (i.e., ADC values saturate at the maximum value of ADC), the ultrasound image is often significantly degraded, for example by distortion, artifacts, clipping, etc. Thus, the settings for the analog amplification are typically a compromise between a cost associated with a signal that is too low (i.e., small dynamic range and decreased signal-to-noise ratio) and the risk of encountering or exceeding a signal that is too high (e.g., clipping).
Typically, a single setting is used for the analog amplification, which setting must accommodate a broad variety of imaging conditions (e.g., varying patients, different anatomy, etc.). Consequently, a significant amount of the total dynamic range is sacrificed to avoid ADC saturation. It is not possible to reliably detect saturation after summing the individual channel contributions because it is possible, and in fact likely, that only some of the ADC channels actually saturate while other ADC channels do not. Thus, the beam sums are typically well below the maximum possible sum (i.e., maximum ADC value per channel times the number of channels).
New approaches that address at least some of the above described saturation issues are desirable.